Thunderstorm Electricity. 635 



The letter to ' Nature ' from which he quotes was in defence 

 of the view (first suggested by J. J. Thomson) that the posi- 

 tive charge of the atmosphere may be due to a prepon- 

 derance of negative ions being carried down in rain, in 

 consequence of their efficiency as condensation nuclei being- 

 greater than that of the positive ions. 



There can, I think, be no doubt that in fine weather 

 regions there is a positive current downwards from the atmo- 

 sphere into the ground in accordance with the direction of 

 the electrical field. The current per sq. cm. of the ground 

 can in fact be measured indirectly from the potential gradient 

 and conducting power of the air, as in the experiments of 

 Gerdien and others, or more directly as in my own expe- 

 riments (Proc. Roy. Soc. vol. Ixxx. p. 537, 1908) ; and the 

 measurements by different methods give values which agree. 

 Until there is evidence to the contrary, it is natural to assume 

 that in wet weather regions a compensating process is going 

 on, due to a preponderance of negative electricity being 

 carried down by rain, the electrical current in the atmosphere 

 from the regions of precipitation to the fine weather regions 

 being one of conduction (Proc. Camb. Phil. Soc. vol. xiii. 

 ]). 3(53) in the upper layers. It may eventually turn out that 

 there is not the required excess of negatively charged rain, 

 and that we shall be compelled to look to some other, possibly 

 cosmical, source for the supply of negative electricity. 



Both TClster and Geitel, and also Gerdien, have, however* 

 found such an excess of negatively charged rain. If further 

 investigations should confirm the apparent preponderance of 

 negative rain found in those earlier experiments, then the supe- 

 rior efficiency of the negative ion as a nucleus may, I think, 

 be looked upon as one possible factor in bringing about this 

 preponderance. It is unfortunately exceedingly difficult in 

 measurements of the charge carried down by rain in heavy 

 showers and thunderstorms to avoid spurious effects. 



In the only paper where I have discussed the possible 

 factors causing strong electrical fields accompanying heavy 

 showers or thunderstorms (' Nature/ vol. lxviii.p. 102, 1003), 

 I pointed out that the difference between positive and negative 

 ions will not of itself account for intense fields. I should 

 like to quote the passage — 



" The following are possible factors in the production of 

 the intense electrical fields which accompany heavy showers — 



" A less degree of supersaturation is required to make water 

 condense on the negative than on the positive ions (C T. R. 

 Wilson, Phil. Trans, vol. cxciii. p. 289) . Thus if condensation 

 takes place from the supersaturated condition, the drops 





